EGC sizing for PV

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shivam.pandey

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Chula Vista
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Solar design engineer
I am working on a PV project and need to size EGC for circuit after DC combiner box. Module Isc is 13.95A. Considering 8.4% bifacial gain effective Isc would be 15.12A. Now per (690.8(A)(1)(1)) Isc would become 18.90A. Now again per (A) (690.8(B)(1)), Isc*1.25 would be 23.63A. We have 12 strings paralleled and total current from the output of the combiner would be 12*23.63=283.5A
Isc= 13.95A
With bifacial gain = 13.95*1.084= 15.12A
Total current at combiner box = 12 strings * 15.12= 181.44A
Now per NEC 181.44*1.56=283.05A

Should I use 300A for the OCPD as this is the next standard OCPD size for EGC sizing or use max disconnect rating of DC combiner box i.e 400A (Combiner is rated for the 400A with 400A disconnect switch).
With 300A, EGC would be 2 AWG and with 400A would be 1 AWG for aluminum conductor.
 
ggunn said:
What do you mean by "ahead"? Terms like that can get confusing with PV systems since operational current flows one way and fault current can flow the other.
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Apologies, I mean after the DC combiner output towards inverter.
No OCPD between DC combiner bus and inverter DC input terminal. See attached image.
Strings combined in DC bus of DC combiner then output current goes to DC switch in the DC combiner and output of DC combiner connected to DC side of Inverter with a DC disconnect inbuilt in the inverter. DC switch in DC combiner is rated for 400A.

DC EGC.jpg
 
Yes you can size it off 300A, because that is what you calculated per 690.9(B). 690.45 says explicitly that you can use this as an "assumed overcurrent device ... when applying table 250.122."
 
jaggedben said:
Yes you can size it off 300A, because that is what you calculated per 690.9(B). 690.45 says explicitly that you can use this as an "assumed overcurrent device ... when applying table 250.122."
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So the PV source conductors all would have to be kept separate to be able to take advantage of 250122(C)?
 
jaggedben said:
690.45 says explicitly that you can use this as an "assumed overcurrent device ... when applying table 250.122."
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2020 NEC 690.45 says you can do that when "When no overcurrent protective device is used in the circuit . . ." Does the fact that each string is fused at 25A (if I understand the drawing correctly) mean that this does not apply?

I guess the question is whether "the circuit" may be taken to mean just the wires between the combiner bus and the inverter terminals, or whether "the circuit" must be taken to include all the strings.

Cheers, Wayne
 
wwhitney said:
2020 NEC 690.45 says you can do that when "When no overcurrent protective device is used in the circuit . . ." Does the fact that each string is fused at 25A (if I understand the drawing correctly) mean that this does not apply?

I guess the question is whether "the circuit" may be taken to mean just the wires between the combiner bus and the inverter terminals, or whether "the circuit" must be taken to include all the strings.

Cheers, Wayne
Click to expand...

Well, in this case it makes no difference: 25A×12=300A. 🙂

I've always understood the combined output circuit to be its own circuit, like a feeder is different from a branch circuit. It is too bad IMO they removed the distinction between source and output circuits; it used to be pretty clear they are different circuits by definition. Different current, different conductor size, OCPDs in between... there is no where else the code wouldn't treat these as separate circuits for the purpose of 250.122. Also the 25A string OCPDs do not protect the combined output from overcurrent. They protect the individual strings from the combined output.
 
ggunn said:
So the PV source conductors all would have to be kept separate to be able to take advantage of 250122(C)?
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Not quite sure what you mean. Multiple source circuits in a raceway can take advantage of 250.122(C) I think. Put an output circuit in with them and the EGC has to be sized for the output circuit IMO.
 
jaggedben said:
Not quite sure what you mean. Multiple source circuits in a raceway can take advantage of 250.122(C) I think. Put an output circuit in with them and the EGC has to be sized for the output circuit IMO.
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Scenario A : 10 PV source circuit conductors in a single conduit. The EGC only needs to be sized to a single circuit because the probability of more than one circuit failing simultaneously is low.

Scenario B: 10 PV source circuits are combined onto a single set of conductors with no OCPD on the combined circuit. The fault current from a single source circuit hasn't changed and neither has the probability of more than one faulting simultaneously, so why should the size of the EGC have to change?
 
ggunn said:
Scenario B: 10 PV source circuits are combined onto a single set of conductors with no OCPD on the combined circuit. The fault current from a single source circuit hasn't changed
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No, but the fault current from a single fault between two wires is now 10 times larger.

Cheers, Wayne
 
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